1. Field of the Invention
This invention relates to a light-scanning optical apparatus and, more particularly, it relates to a light-scanning optical apparatus adapted to cause a light beam emitted from a light source to strike a deflection plane of an optical deflector with a predetermined angle of incidence in a plane intersecting the optical axis along the sub-scanning direction (hereinafter referred to as sub-scanning section) and the light beam deflected/reflected by the optical deflector to scan a surface to be scanned. A light-scanning optical apparatus according to the invention can suitably be used for an image-forming apparatus such as a laser beam printer or digital copying machine.
2. Related Background Art
Conventional oblique incidence optical systems adapted to cause a light beam emitted from a light source to strike a deflection plane of an optical deflector with a predetermined angle of incidence in the sub-scanning section typically comprise as the focussing optical system (fθ lens system) a cylindrical lens having power of converging the light beam only in the main-scanning direction as proposed in Japanese Patent Application Laid-Open No. 9-96773.
Additionally, such systems typically employ a cylindrical mirror having power of converging the light beam only in the sub-scanning direction as the tilt correcting operation system for correcting the tilt of the reflection plane of the optical deflector.
FIG. 12 of the accompanying drawings is a schematic perspective view of a principal portion of a light-scanning optical apparatus proposed in the above patent document.
Referring to FIG. 12, the light beam emitted from a semiconductor laser 41 is transformed into a slightly divergent light beam by a condenser lens 42 and limited by an aperture stop 43 before entering a cylindrical lens 44 of incidence system having power of converging the light beam only in the sub-scanning direction. The slightly divergent light beam entering the cylindrical lens 44 is converged in the sub-scanning direction and then enters second and first cylindrical lenses 46 and 47 of an fθ lens system 53 having power of converging the light beam only in the main-scanning direction by way of a fold mirror 45 to become focussed and form a linear image (extending in the main-scanning direction) near the deflection plane 48a. Note that the light beam strikes the deflection plane 48a with a predetermined angle relative to a plane perpendicular to the axis of rotation of the optical deflector 48 (plane of rotation of the optical deflector 48) in the sub-scanning section that contains the axis of rotation of the optical deflector 48 and the optical axis of the focussing optical system 52.
On the other hand, the light beam is transmitted through the second and first cylindrical lenses 46 and 47 of the fθ lens system 53 without being modified (and hence as slightly divergent light beam) in a plane intersecting the optical axis in the main-scanning direction (hereinafter referred to as main-scanning section).
The light beam deflected/reflected by the deflection plane 48a of the optical deflector 48 is then led to the surface of a photosensitive drum 50 that is a surface to be scanned by ways of the first and second cylindrical lenses 47 and 46 and a cylindrical mirror 49 having power of converging the light beam only in the sub-scanning direction so that it scans the surface of the photosensitive drum 50 in the direction of arrow B (main-scanning direction) as the optical deflector 48 is driven to rotate in the sense of arrow A. As a result, an image is recorded on the surface of the photosensitive drum 50 operating as recording medium.
Note that, in FIG. 12, both the lens surface 47a of the first cylindrical lens 47 facing the surface of the photosensitive drum 50 and the lens surface 46a of the second cylindrical lens 46 facing the optical deflector 48 are planes.
In recent years, there has been a strong demand for scanning optical systems adapted to scan the surface to be scanned 50 at high speed. While the demand may be met by increasing the number of deflection planes 48a of the polygon mirror 48 without using a large polygon mirror 48, each of the deflection plane of a polygon mirror 48 having such a large number of deflection planes 48a inevitably shows a small area and a small deflection angle. Thus, such a polygon mirror is accompanied by the problem of a narrow scanning width on the surface to be scanned 50 when the light beam shows a width smaller than the facet width of the deflection plane 48a. Then, the entire surface of the deflection plane 48a has to be utilized to deflect/reflect the light beam emitted from the light source which is typically a semiconductor laser 41. This requirement of using the entire surface of the deflection plane 48a is met by using an overfilled system where the width of the light beam is sufficiently larger than the facet width of the deflection plane 48a. 
However, the overfilled system has a problem that the intensity of the light beam coming from the deflection plane 48a can vary as a function of the deflection angle of the deflection plane 48a. To alleviate this problem, the light beam emitted from the semiconductor laser 41 is required to squarely strike the optical deflector 48 by using an arrangement of causing the light beam to pass through the fθ lens system 53, which is part of the incidence system, both when striking the optical deflector 48 and when coming back from the optical deflector 48 (double pass arrangement) and/or causing the light beam emitted from the light source 1 to strike the deflection plane 48a substantially from the center of the deflection angle of the optical deflector 48 (front incidence arrangement). Then, it is practically impossible to make the fθ lens system 53 or the light-scanning optical apparatus show a flat profile. As a result, an image forming apparatus comprising such an fθ lens system and a light-scanning optical apparatus will not only be costly but also bulky.
Additionally, the use of a double pass arrangement gives rise to a problem of wavefront aberration due to the fact that the light beam passes through the fθ lens system 53 twice.
To avoid the above problems and realize a compact light-scanning optical apparatus, the one shown in FIG. 12 (Japanese Patent Application Laid-Open No. 9-96773) is so arranged as to make the fθ lens system 53 not to have power in the sub-scanning direction and the light beam passing therethrough to advance without being refracted in the sub-scanning direction, while the light source 41 is arranged at a position close to the optical axis of the fθ lens system 53 to make the image forming apparatus a very flat one. Then, the wavefront aberration is reduced in the sub-scanning direction.
However, the above described known light-scanning optical apparatus is accompanied by the following problems that have to be dissolved.
(1) The first and second cylindrical lenses 47 and 46 are costly relative to spherical lenses. Particularly, the surface facing the optical deflector 48 of the first cylindrical lens 47 is required to have a large curvature to make it difficult to prepare and costly.(2) An optical system using a cylindrical mirror 49 does not provide any choice in terms of the fold angle of the cylindrical mirror 49. Once designed, the fold angle becomes invariable and both the cylindrical mirror 49 and the fθ lens system 53 have to be redesigned to modify the fold angle.(3) As the light beam enters the first and second cylindrical lenses 47 and 46, part of the light beam trying to enter either of the lenses is reflected by the surface thereof to get to a central part of the image and adversely affect the image quality.